1. Field of the Invention
The invention relates to an X-ray examination apparatus, comprising an X-ray source for forming an X-ray image by irradiation of an object by means of X-rays, an X-ray image intensifier for convening the X-ray image into an optical image, and an image pick-up device for forming an electronic image signal from the optical image, which image pick-up device comprises an exposure control device for applying a control signal, derived from the electronic image signal, to a control input of an amplifier of the image pick-up device, which exposure control device comprises a selector for selecting at least one measuring field in the optical image, a maximum detector for detecting a maximum level derived from a maximum value of signal amplitudes of the image signal corresponding to at least one measuring field, a minimum detector for detecting a minimum level derived from a minimum value of signal amplitudes of the image signal corresponding to at least one measuring field, and a mean value detector for detecting a mean level derived from a mean value of signal amplitudes of the image signal corresponding to at least one measuring field.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from German Auslegeschrift DE 26 10 845.
The known X-ray examination apparatus utilizes the mean value of the image signal in the measuring field as an X-ray control signal for controlling a power supply for the X-ray source in order to control the exposure time, or the X-ray pulse duration, so as to achieve adequate image contrast and image brightness and to limit the X-ray dose whereto a patient to be examined is exposed. The known X-ray examination apparatus comprises a selector switch for selecting one of six predefined measuring fields. Furthermore, in the known X-ray examination apparatus the relative difference between a maximum signal amplitude and a minimum signal amplitude is used for the control signal for adjustment of the signal amplification of the electronic image signal. The amplifier derives an amplified video signal from the electronic image signal, which video signal is applied to a monitor for display of an image. In the case of such control of the signal amplification, involving only the minimum and the maximum signal amplitudes without taking into account notably the mean signal amplitude, sudden transitions often occur in the image brightness of an image displayed on the monitor in the event of variation of objects reproduced in the measuring field of the optical image. Notably in the case of dynamic imaging, for example during fluoroscopy, sudden variations occur in image brightnesses of the successive images reproduced on the monitor.